Well depends on the dog. I HATE no pull harnesses as they do nothing but pull the dog out of alignment and can cause damage to the spine.... but off topic..
As far as collars go it depends on your dog. I like martingales and what I would do is put the dog on a martingale collar, go to a park or somewhere you can make a correction for getting excited around dogs and the second his body language changes when he sees another dog give a good pop and release on the leash and say NO. Make sure the martingale is fitted correctly, when you pull tight there should be some chain still between the nylon. If the nylon on both sides of the neck comes together when you pull tight you need to adjust the collar smaller. After some good placed correction your dog still ignores you then it's time to step up to a prong collar. You need a 2.25 mm 14" prong collar fitted correctly I will post a video that shows how to do this. (a martingale collar should fit the same as a prong collar with the exception of having to slide it on the dog then adjust it smaller) I would suggest putting the prong on a dead ring the first time you have to make a correction, the video explains this. If the results are not what you want then move it to the live ring of the collar and give a well placed correction.
How to Fit a Prong Collar
This should help you correct the reactive problem.